1. Field of the Invention
The present invention relates to a method of laser-annealing an amorphous silicon film or a crystalline silicon film formed on an insulating substrate made of glass or the like to crystallize the film or to improve the crystallinity.
2. Description of the Related Art
In recent years, a technique has been widely researched in which laser-annealing is conducted on an amorphous silicon film or a crystalline silicon film (a silicon film having crystallinity such as polycrystal or microcrystal, which is not mono-crystal) formed on an insulating substrate made of glass or the like, that is, a non-monocrystal silicon film, to crystallize those films or to improve the crystallinity.
The crystalline silicon film formed by laser-annealing, for its high mobility, has been widely employed for a monolithic type liquid-crystal electro-optic device, etc., in which a thin-film transistor (TFT) is formed using the crystalline silicon film, and TFTs for pixel driving and drive circuits are then fabricated, for example, on a single glass substrate.
Also, a method has been preferred to employ in which pulsed laser beams such as an excimer laser is processed by an optical system so as to be formed into beams having a square spot of several cm.sup.2 in cross section or beams having a linear shape of several mm width.times.several tens cm, and laser-annealing is conducted by scanning the laser beam thus processed (while the radiation position of the laser beams is moved relatively with respect to a surface to be irradiated), because it is improved in productivity and excellent industrially.
In particular, the use of the linear laser beam makes the productivity high because laser beams can be irradiated on the overall surface to be irradiated by the scanning operation conducted only in a direction perpendicular to the linear direction, which is different from a case in which spot-like laser beams that require the right and left scanning operation as well as the forward and backward scanning operation are used.
There arise several problems in conducting laser-annealing on the non-monocrystal silicon film by scanning spot-like or linear laser beams emitted from a pulsed laser beam source thereon.
For example, in the case where laser-annealing is conducted in the air, there arises such a problem that impurities of carbon contained in the air and other materials are liable to be mixedly inserted into the film, to thereby deteriorate the various characteristics such as the quality, the crystallinity or the mobility of the crystalline silicon film which has been annealed.
Also, in the case where laser-annealing is conducted by scanning beams which are spot-shaped or linear on the surface to be irradiated in a vacuum atmosphere or inactive gas atmosphere such as nitrogen, the following problems are caused in comparison with annealing in the air.
1) The crystallinity is deteriorated. That is, a high crystallinity cannot be obtained without largely increasing the energy density of a laser beam in comparison with annealing in the air.
2) The uniformity in the film of the crystal is deteriorated. Locations where the crystallinity is high and locations where the crystallinity is low are distributed in the film. For example, in the case where linear laser beams are scanned in a direction perpendicular to the linear direction of the beam, locations where the crystallinity is high and locations where the crystallinity is low appear in the form of a stripe pattern on the film surface. Accordingly, in the case where a plurality of thin-film transistors are manufactured using the fabricated crystalline silicon film, a variety of characteristics such as a threshold value or mobility are different depending on a position of the thin-film transistor on a substrate.
3) The use efficiency of an energy is deteriorated. For the purpose of enhancing the crystallinity, the energy density of laser must be increased. As the energy density is increased, the power consumption is also increased. In addition, the entire laser irradiating device including a laser oscillator and a circuit, a gas and an optical systems is largely consumed, resulting in the increased costs of a manufactured device. Also, although the crystallinity is increased as the energy density of laser is increased, an entire film which has been subjected to laser-annealing is remarkably roughened, thereby making it hard to manufacture the device by processing the film.